Conventional electric motors, both rotary and linear, have very poor torque and force density. For heavy lifting applications, mechanical means such as gears and screws are more typically used. Fluidic devices including pneumatic and hydraulic systems are also used. These mechanical methods generally involve noise, wear, backlash, poor shock tolerance, and high reflected inertia. The fluidic methods tend to increase system complexity due to the addition of a fluid system. Fluid systems are also harder to control than electric systems. Due to seal wear, the fluid methods are unreliable and can contaminate sensitive environments when the working fluid leaks.
Most prior art linear motors operating on the principle of magnetism, however, include permanent magnets or are classified as inductance machines. U.S. Pat. No. 4,864,169, also incorporated herein by this reference, discloses a linear reluctance motor but it is configured such that the magnetic flux produced extends in the direction of the actuation axis. Such a design, however, results in a fairly low force density.
In general, variable reluctance motors include a stationary part (stator) which includes coil(s), each of which has an associated “pole”, typically made of some form of iron. The combination of a coil and its “poles” is known as a “phase”. A moving part (rotor) also has “poles”, and when a phase is energized, the rotor tends to move so as to align its “poles” with those of the stator. When aligned with this energized phase, the other two (or more) phases are so arranged that their poles are offset in one direction or the other. By energizing the phases in the correct sequence, the motor moves or revolves. See e.g. U.S. Pat. No. 3,992,641, which discloses a rotor-stator alignment arrangement for a poly-phase disk motor with improved starting torque regardless of position, which is incorporated herein by reference.
As in any electric motor, torque results from the electromagnetic shear force acting between the rotor and the stator. A common term in motor design is the operating “shear pressure”, i.e. the shear force per unit area. In a variable reluctance motor, the shear pressure goes up with the flux-density squared, but the maximum pressure is sharply limited by the saturation flux-density of the poles and flux return (typically made of an iron alloy).
A polyphase disk reluctance rotary motor is shown in U.S. Pat. No. 3,992,641 incorporated herein by this reference. The motor therein described uses interleaved disks and radial teeth to provide higher torque than conventional motors, however, the torque is still sharply limited due to the use of thick, self-supporting disks and large gaps between disks.